86 The Scottish Naturalist. 



periodical return, like the bird from the sunny south, for the 

 reproduction of young. The herring was once believed to 

 migrate frofti the extreme north, and to divide into two 

 columns, one of these advancing towards Iceland, and skirting- 

 the shores of America, the other taking an opposite direction 

 along the shores of Norway, and furnishing a branch to the Baltic, 

 while a large body spreading out was supposed to visit the 

 coasts of France and Great Britain. It is now, however, pretty 

 well understood that these fish merely retire into deeper 

 water, reappearing at the stated period of reproduction, and 

 never entirely leaving our coasts ; and this I believe will apply 

 to the Pilchard, the Mackerel, and many others. The fish 

 which visit our rivers at stated times, such as the Salmon, 

 Salmon-trout, &c., never leave our own shores, but come as 

 it were (if I may so express myself in the case of a fish) 

 inland for the purpose of breeding, much the same as many 

 of our coast birds do in the breeding season, and which may be 

 considered in their case also as merely a local or partial migra- 

 tion. 



The Eel, which is also a local migrant, has this peculiarity, 



that instead of the adult fish ascending our rivers, he desce?ids 



for reproduction, as also to obtain during the winter months a 



more suitable temperature in the brackish water of the estuary, 



which is higher in that respect than either pure fresh water or 



the sea ; it being, as the late Mr. Yarrell remarks in his valuable 



work on British Fishes, a well-known law in chemistry, that 



when two fluids of different densities come in contact, the 



temperature of the mixture is elevated for a time, in proportion 



to the difference in density of the two fluids, from mutual 



penetration and condensation. Such a mixture, is constantly 



taking place at the mouths of rivers that run into the sea, 



and the mixed water maintains a temperature two degrees 



warmer than that of the river or of the sea. So strong is the 



instinct of migration implanted in the eel, that in cases where 



they have been introduced into ponds not having free egress 



to the brackish water, they have been known to travel over land in 



the attempt to reach it. Mr. Yarrell quotes a wonderful instance 



of this mentioned by Dr. Hastings in his " Illustrations of the 



Natural History of Worcestershire " (p. 134), wherein he states 



that " a relation of the late Mr. Perrott was out in his park with 



the keeper, near a large piece of water, on a very beautiful 



evening, when the keeper drew his attention to a fine eel 



